The ability to detect various chemicals and to use this information to make feeding decisions is a defining feature of the taste system from humans to insects. In Drosophila, a large family of Gustatory receptor (Gr) genes encodes divergent receptors that are expressed in taste neurons. Despite ongoing efforts, we know little about the ligand response profiles of individual Gr proteins in any insect. Gr proteins represent a novel family, distantly related only to insect Odorant receptor (Or) proteins, and thus lack the advantage of structural or functional comparisons with equivalent groups. The same feature of exclusivity in insects and arthropods, however, makes Grs attractive targets for developing novel strategies of insect control. In preliminary studies, we have developed an in vivo functional expression system in which to identify the response properties of individual insect Grs. The overarching objective of this proposal is to elucidate functional properties of individual taste receptors in Drosophila and the malaria mosquito Anopheles gambiae, and their role in instructing feeding behavior. We propose to systematically characterize the tuning profiles of selected taste receptors of the fruit fly and the mosquito by taking advantage of our ectopic expression system. The experiments are designed to reveal fundamental features of Gr function including the activation and inhibition profiles of individual receptors, the extent of functional overlap between receptors, and the relationship between receptor sequence and function. The approach to address these questions will be to: 1) Identify the tuning profiles of internally expressed Drosophila taste receptors, and evaluate their roles in feeding behavior by genetic analysis, 2) Characterize novel cross-modality interactions between Drosophila sweet taste receptors and bitter tastants, and examine the functional role of sweet taste neuron inhibition in feeding behavior, and 3) Identify activators and inhibitors of putative sweet taste receptors of An. gambiae and characterize the response profiles of endogenous sweet taste neurons in the mosquito proboscis. The proposed research is innovative because it employs a novel in vivo ectopic expression system for decoding Gr function and combines it with molecular genetics, electrophysiological, and behavioral analyses of gustatory function. The research is significant because functional analysis of Gr genes will contribute to our understanding of the fundamental principles by which tastants are encoded across a diverse repertoire of receptors and how mechanisms of tastant detection translate to food selection. Additionally, our studies will shed light on molecular and cellular mechanisms of sugar detection in mosquitoes, which may be applicable for improving sugar-baited traps that are used in mosquito surveillance and control strategies.